The invention relates to an alarm circuit designed to detect interruptions in the electrical connection between a low impedance signal device and a signal processing device. The signal processing device has been provided with an amplifier at its input which has a higher input impedance than the impedance of the signal device as well as with a supply circuit to supply an auxiliary dc current to the connection between the signal device and the signal processing device. An alarm is activated as soon as a predetermined upper limit of the amplifier input voltage has been exceeded.
There are no restrictions with respect to the design of either the signal device or the signal processing device, as long as the above prerequisites are observed with respect to the input impedance of the respective devices. The signal device must have a comparatively low impedance and the signal processing device a comparatively higher impedance. For example, a microphone can be used as the signal device, while a microphone amplifier may be used as a signal processing device. However, the circuit is preferably used with a system applied to sense electrical voltages generated by a patient. Configurations of this kind are ECG, EEG and EMG devices. During the sensing of electrical voltages generated by the body of a patient, an electrical amplifier is attached to the patient via one or several electrodes and electrode lines. The satisfactory operation of a circuit of this type depends essentially on the existence of a proper connection between a patient and an amplifier input. As soon as the electrical circuit between the signal device (electrode) and the signal processing device (evaluation device with or without screen) is interrupted, reliable sensing of voltages generated by the patient is no longer ensured. As a result, signals measured during this time period may be misinterpreted. Therefore, prior art circuits include means to detect and indicate interruptions in the electrical input circuit of the amplifier.
Known circuits used in ECG measurements monitor the upper limits of the source impedance (patient circuit), which is connected to the amplifier input. Under normal conditions the source impedance lies below the upper limit. The source impedance is comprised of the impedance of the supply lines between the electrode and the signal processing device as well as the tissue impedance and the electrode transfer impedance. During interruptions, such as may occur when an electrode is removed from the patient, the impedance value exceeds the predetermined upper limit. The alarm device interprets this increase as in impedance a line interruption.
In prior art devices, the source impedance is monitored by feeding an electrical current of approximately constant amplitude into the input of the amplifier which comprises the connection point between the signal device and the signal processing device. The voltage measured at the input of the amplifier can then be used to determine the magnitude of the source impedance and an alarm will be generated when a predeterminable limit is exceeded. In order not to distort the signals that are sensed from the patient, which range between 0.1 Hz to 1 kHz frequency, prior art devices employ a signal with a frequency far exceeding this range. For this purpose, a dc signal (Siemens brochure "System SIRECUST 400" E 3331, order number M-E 331/2077, article number 73 70 059 E 2254) or an ac signal of around 12 kHz (Siemens brochure "System SIRECUST 400", E 331, order number M-E 331.2077, article number 73 70 000 E 2253) may be used. However, both alternatives present disadvantages as described below.
For example, when supplying an approximately constant auxiliary dc current, the following disadvantages occur:
(a) Changes in the source impedance may lead to an erroneous voltage, superimposed on the voltage to be sensed.
(b) A dc current supplied via the electrodes increases the polarization voltage at the electrodes, that is to say, the voltage flowing between the electrode metal (usually Ag/AgCL) and the electrode contact gel. In turn this polarization voltage can be modulated by movements of the patient.
Both effects (a) and (b) can create motion artefacts which interfere with the evaluation process and/or the screen display.
(c) The dc current changes the electrode surface. This means, that the surface is more readily prone to build-ups or modifications (chemical changes) of the metal used for the electrode. This is certainly true for Ag/AgCl which is commonly used.
(d) The dc current leads to ion migration through the skin of the patient. It also changes the chemical composition within the tissue and the contact electrolytes usually located between the skin and the electrode surface.
The severity of the adverse effects mentioned above are increased in proportion to the magnitude of the applied auxiliary dc current. Therefore, it is recommended that low current values be used. However, this current value is limited to a certain minimum due to the insulation resistance of the total configuration and the amplitude of the source voltage to be processed. The type of insulation selected for the supply lines determines to a large degree the insulation resistance.
In order to prevent these disadvantages the alternate method mentioned above is used. An auxiliary ac current of relatively high frequency, e.g. 12 kHz is supplied. However, this approach has the disadvantage of an already low insulation impedance due to the line capacitance. Therefore only a relatively low impedance limit can be monitored as source impedance. In addition, the ac current feed-in can lead to interruptions during the presence of high frequency interference voltages. High frequency interference voltages might occur for example in connection with high frequency surgery.